Three-phase brushless motor state identification

ABSTRACT

A method for determining the state of a brushless motor having first, second and third phases, in some embodiments, comprises: decoupling said motor from a power source; determining whether said motor is rotating or non-rotating; if the motor is rotating, determining a first phase voltage state relative to a common voltage and a second phase voltage state relative to the common voltage, said first phase and second phase voltage states determined when a third phase voltage is within a predetermined range of said common voltage; and if the first phase voltage state and the second phase voltage state are the same, repeating said determination as to whether the motor is rotating or non-rotating.

BACKGROUND

Three-phase brushless motors are used in a wide variety of applications.In order to drive such motors in a stable manner at all times, it isnecessary to be able to accurately determine the state of the motor—thatis, whether the motor is stopped or rotating, and, if rotating, whetherit is rotating in a positive or reverse direction. Some techniques fordetermining the motor state are known, but these techniques oftenproduce inaccurate results because they fail to account for variousexternal influences, such as noise. The consequences of such inaccuratedetermination of motor state can be severe, including slow motor startupand overheating resulting in significant motor damage. Thus, techniquesfor more accurately identifying motor state are desired.

SUMMARY

At least some embodiments are directed to a method for determining thestate of a brushless motor having first, second and third phases,comprising: decoupling said motor from a power source; determiningwhether said motor is rotating or non-rotating; if the motor isrotating, determining a first phase voltage state relative to a commonvoltage and a second phase voltage state relative to the common voltage,said first phase and second phase voltage states determined when a thirdphase voltage is within a predetermined range of said common voltage;and if the first phase voltage state and the second phase voltage stateare the same, repeating said determination as to whether the motor isrotating or non-rotating. Some or all of these embodiments may besupplemented using one or more of the following concepts, in any orderand in any combination: further comprising if the first phase voltagestate and the second phase voltage state are the same, repeating saiddetermination of the first phase and second phase voltage states;wherein said determination of the first phase and second phase voltagestates is repeated no more than a predetermined number of times;wherein, if said determination of the first phase and second phasevoltage states is repeated said predetermined number of times,determining that the state of said motor is non-rotating; furthercomprising if the first phase voltage state is low and the second phasevoltage state is high, and if said first and second phase voltage statesare determined at a time when the third phase voltage is rising, thendetermining that the state of said motor is positive rotation; furthercomprising if the first phase voltage state is high and the second phasevoltage state is low, and if said first and second phase voltage statesare determined at a time when the third phase voltage is rising, thendetermining that the state of said motor is reverse rotation; furthercomprising if the first phase voltage state is low and the second phasevoltage state is high, and if said first and second phase voltage statesare determined at a time when the third phase voltage is falling, thendetermining that the state of said motor is reverse rotation; furthercomprising if the first phase voltage state is high and the second phasevoltage state is low, and if said first and second phase voltage statesare determined at a time when the third phase voltage is falling, thendetermining that the state of said motor is positive rotation; whereindetermining whether the motor is rotating or non-rotating comprises:providing two comparators, each of said comparators receiving the samephase voltage and each of said comparators receiving a differentlybiased version of the common voltage; if both outputs of said twocomparators are the same polarity, determining that the state of saidmotor is rotating.

At least some embodiments are directed to a method for determining thestate of a brushless motor having first, second and third phases,comprising: determining whether said motor is rotating or non-rotating;if the motor is rotating, determining a first phase voltage staterelative to a common voltage and a second phase voltage state relativeto the common voltage, said first phase and second phase voltage statesdetermined when a third phase voltage is approximately equal to saidcommon voltage; and if the first phase voltage state and the secondphase voltage state are the same, repeating said determination of thefirst phase voltage state and the second phase voltage state. Some orall of these embodiments may be supplemented using one or more of thefollowing concepts, in any order and in any combination: furthercomprising disconnecting the motor from a power source prior toperforming said determination steps; wherein, if said determination ofthe first phase and second phase voltage states has been repeated apredetermined number of times, determining that said state of the motoris non-rotating; wherein, if the first phase voltage state and thesecond phase voltage state are the same, omitting repetition of saiddetermination as to whether the motor is rotating or non-rotating;further comprising if the first phase voltage state is low and thesecond phase voltage state is high, and if said first and second phasevoltage states are determined when the third phase voltage is rising,then determining that the state of said motor is positive rotation;further comprising if the first phase voltage state is high and thesecond phase voltage state is low, and if said first and second phasevoltage states are determined when the third phase voltage is rising,then determining that the state of said motor is reverse rotation;further comprising if the first phase voltage state is low and thesecond phase voltage state is high, and if said first and second phasevoltage states are determined when the third phase voltage is falling,then determining that the state of said motor is reverse rotation;further comprising if the first phase voltage state is high and thesecond phase voltage state is low, and if said first and second phasevoltage states are determined when the third phase voltage is falling,then determining that the state of said motor is positive rotation;wherein determining whether the motor is rotating or non-rotatingcomprises: providing two comparators, each of said comparators receivingthe same phase voltage and each of said comparators receiving adifferently biased version of the common voltage; if both outputs ofsaid two comparators are the same polarity, determining that the stateof said motor is rotating.

At least some embodiments are directed to a system for determining thestate of a motor, comprising: a motor that is associated with first,second and third phases and that is driven by a motor driving circuit; acontrol circuit that couples to and controls the motor driving circuit;a first comparator that receives a first biased version of a commonvoltage at one input and a first phase voltage, a second phase voltage,or a third phase voltage at another input, an output of said firstcomparator provided to the control circuit; and a second comparator thatreceives a second biased version of the common voltage at one input andthe first phase voltage, the second phase voltage, or the third phasevoltage at another input, an output of said second comparator providedto the control circuit; wherein, if the outputs of the first and secondcomparators are both positive or are both negative, the control circuitdetermines that the motor is rotating, wherein, if the motor isdetermined to be rotating, the control circuit determines a first phasevoltage state relative to the common voltage and a second phase voltagestate relative to the common voltage, said first phase and second phasevoltage states determined when a third phase voltage is approximatelyequal to said common voltage, and wherein, if the first phase voltagestate and the second phase voltage state are the same, the controlcircuit repeats said determination of the first phase voltage state andthe second phase voltage state. Some or all of these embodiments may besupplemented using one or more of the following concepts, in any orderand in any combination: wherein, if the first phase and second phasevoltage states are the same, the control circuit repeats saiddetermination of whether said motor is rotating; wherein first andsecond bias voltages associated with said first and second biasedversions of the common voltage have opposing polarities and haveabsolute values that are less than: an absolute value of a maximum firstphase voltage; an absolute value of a maximum second phase voltage; andan absolute value of a maximum third phase voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

There are disclosed in the drawings and in the following descriptiontechniques for identifying the state of a three-phase, brushless motor.In the drawings:

FIG. 1 is a block diagram of a motor state identification circuit.

FIG. 2A is a flow diagram of a method for identifying motor state.

FIG. 2B is a flow diagram of a sub-method of the method depicted in FIG.2A.

FIG. 3 is a table describing the manner in which three-phase voltagesignals are interpreted.

FIG. 4 is a two-part graph of waveforms depicting positive rotationphase voltages and reverse rotation phase voltages.

FIG. 5 is a flow diagram of another method for identifying motor state.

FIG. 6A is a flow diagram of yet another method for identifying motorstate.

FIG. 6B is a flow diagram of still another method for identifying motorstate.

It should be understood, however, that the specific embodiments given inthe drawings and detailed description thereto do not limit thedisclosure. On the contrary, they provide the foundation for one ofordinary skill to discern the alternative forms, equivalents, andmodifications that are encompassed together with one or more of thegiven embodiments in the scope of the appended claims.

DETAILED DESCRIPTION

Disclosed herein is a technique for determining the rotational state ofa three-phase, brushless motor. The technique generally involves the useof a circuit, described below, that first determines whether the motorin question is rotating or non-rotating. If the motor is determined tobe rotating, the technique involves determining the direction in whichthe motor is rotating—that is, either a positive rotation or a reverserotation. The direction of rotation is determined by identifying thevoltage polarities of two of the three phases when the voltage of thethird phase is approximately equal to a common voltage (e.g., amid-potential voltage). If, at the time that the third phase voltage isapproximately equal to the common voltage, the voltage states of thefirst and second phases are the same (i.e., both HIGH or both LOW), themotor is determined to be in an abnormal state. Because this abnormalstate may be caused by noise that may later resolve, the techniquedescribed above is repeated, beginning either with the determination ofwhether the motor is rotating, or with the determination of whether themotor is rotating in a positive or reverse direction. If no abnormalresult is produced, the motor is identified as being either in apositive or reverse rotation. However, if an abnormal result is producedagain, the technique described above is repeated again, and the processcontinues to repeat until either the abnormal result no longer occurs oruntil a predetermined number of repetitions have occurred, at whichpoint the motor is said to be non-rotating, and the technique iscomplete.

FIG. 1 is a block diagram of a motor state identification circuit 100.The circuit 100 includes a motor 102. The motor 102 is a three-phase,brushless motor. The phases are labeled as U, V, and W, in accordancewith convention. The circuit 100 also includes a motor driving circuit104 that provides power to the motor 102 in three phases. The circuit100 further includes a control circuit 106, which controls the motordriving circuit 104. Two comparators 108, 110 provide their outputs tothe control circuit 106. Each of the comparators 108, 110 has aninverting input and a non-inverting input. The non-inverting inputs arecoupled to each other at a common node 112 so that the signal providedto one of the non-inverting inputs is also provided to the othernon-inverting input. The common node 112 couples to three switches 114,116, 118, each of which, in turn, couples to a different phase U, V, orW.

The inverting input of the comparator 108 couples to a bias circuit 120,while the inverting input of the comparator 110 couples to a biascircuit 122. Each of the bias circuits 120, 122 is controlled by thecontrol circuit 106. Each of the bias circuits 120, 122 adds a differentvoltage bias to the common voltage (mid-potential voltage) associatedwith the three-phase motor 102. The common voltage is present at node124 and may be referred to as “COM” in shorthand. In this way, the biascircuit 120 provides a biased version of the COM voltage to thecomparator 108, and the bias circuit 122 provides a biased version ofthe COM voltage to the comparator 110. The degree of bias may be set byan operator of the circuit 100 as may be appropriate and desirable, butin at least some embodiments, the absolute values of the bias voltagesapplied at circuits 120, 122 are less than the absolute values of themaximum voltages produced at the U, V and W phases. Further, in at leastsome embodiments, the bias voltages have opposing polarity, meaning thatthe voltage output by the bias circuit 120 is slightly greater than theCOM voltage while the bias circuit 122 produces a voltage that isslightly lesser than the COM voltage; alternatively, the bias circuit120 produces a voltage that is slightly lesser than the COM voltagewhile the bias circuit 122 produces a voltage that is slightly greaterthan the COM voltage. Although the precise bias applied at the circuits120, 122 may vary, in at least some embodiments, the bias voltagesapplied may be on the order of tens of millivolts. The scope ofdisclosure is not limited to any particular magnitude of bias, however,and any and all suitable bias voltages are contemplated.

The operation of the circuit 100 is best described with respect to theflow diagrams provided in FIGS. 2A and 2B. In at least some embodiments,the control circuit 106 performs some or all steps of the methodsdescribed in FIGS. 2A and 2B, but the scope of disclosure is not limitedas such. The method 200 of FIG. 2A begins at step 202. At step 204, alldrive outputs are shut off, meaning that the motor driving circuit 104ceases to provide power to the motor 102. The motor is still rotating,however, and the direction of this rotation is that which must bedetermined. To this end, the method 200 comprises determining therotational state of the motor (step 206). The control circuit 106determines whether the motor 102 is rotating or not by comparing each ofthe U, V and W phase voltages (inductive voltages) to the biasedvoltages produced by circuits 120, 122. If each such comparison producesa negative value and a positive value, the motor 102 is non-rotating(i.e., stopped). In that case, the method 200 comprises concluding thatthe motor 102 is non-rotating (step 206) and applying electricalcontrols to the motor 102 appropriate for a non-rotating state (step214). However, if any such comparison produces two negative values ortwo positive values, the motor 102 is determined to be rotating, and themethod 200 proceeds to step 208.

For example, assume that the motor 102 is non-rotating. Thus, the phasevoltages at U, V and W (and COM) are all the same—for instance, 0.0 V.Further assume that switch 114 is closed and switches 116, 118 are open.The comparator 108 will compare the 0.0 V from the U phase to the biasedvoltage produced by bias circuit 120. Assuming a bias voltage here of+10.0 mV, the output from the comparator 108 will be 0.0 mV-10.0mV=−10.0 mV. In other words, the output of the comparator 108 will benegative. Similarly, the comparator 110 will compare the 0.0 V from theU phase to the biased voltage produced by the bias circuit 122. Assuminga bias voltage here of −10.0 mV, the output from the comparator 110 willbe 0.0 mV-−10.0 mV=+10.0 mV. In other words, the output of thecomparator 110 will be positive. The process will be repeated for the Vand W phase voltages, with the same polarities produced at the outputsof the comparators 108, 110. Because opposing polarities are produced atthe outputs of the comparators for each of the U, V and W phasevoltages, the motor 102 is determined to be non-rotating. If, however, asingle one of the comparisons has produced two positives or twonegatives, the motor 102 would be determined to be rotating. Forinstance, had the motor been rotating, the U, V and W phase voltages mayhave been such that the outputs of the comparators 108, 110 would benegative and positive for the U phase voltage, negative and positive forthe V phase voltage, and positive and positive for the W phase voltage,signifying that the motor 102 is rotating.

Referring still to FIG. 2A, and assuming that the motor 102 isdetermined to be rotational at step 206, the method 200 comprisesdetermining the direction of the motor rotation (step 208). FIG. 2Bcontains a flow chart of step (or method) 208 describing how thisdirection is determined. Method 208 begins at step 220. At step 222,either of the comparators 108, 110—for purposes of discussion, onlycomparator 108 will be referenced, but comparator 110 would also besuitable—is used to determine the state of the U phase voltage inrelation to the COM voltage (or, in some embodiments, in relation to theCOM voltage plus or minus a small bias voltage, which should be set at anegligible value for purposes of determining rotation direction). If theU phase voltage is determined to be greater than the COM voltage, the Uphase voltage state is said to be HIGH, while if the U phase voltage isdetermined to be less than the COM voltage, the U phase voltage state issaid to be LOW. Accordingly, the method 208 comprises making thisdetermination. If the U phase voltage state is determined to be LOW, themethod 208 comprises waiting until the U phase voltage is approximatelyequal to the COM voltage as the U phase voltage rises (step 224).(Bearing in mind that the voltages applied to the three phase motor 102are sinusoidal, the U, V and W phase voltages rise and fall regularly.)Once the U phase voltage is approximately equal to the COM voltage(i.e., within a predetermined range of 5% of the COM voltage), themethod 208 comprises determining the V and W phase voltage statesrelative to the COM voltage (step 228). (If the step 224 is notcompleted within a predetermined amount of time, the method 208comprises determining that the motor 102 is non-rotating (step 238).) Ifthe V and W phase voltage states are both LOW, or if they are both HIGH,the method 208 includes determining that the motor 102 is in an abnormalstate (step 232). If, however, at step 228 it is determined that the Vphase voltage state is LOW and the W phase voltage state is HIGH, themethod 208 concludes that the motor 102 is in positive rotation (step234). Alternatively, if it is determined that the V phase voltage stateis HIGH and the W phase voltage state is LOW, the method 208 concludesthat the motor 102 is in reverse rotation (step 236).

Referring again to step 222, if the U phase voltage state is determinedto be HIGH, the method 208 includes waiting until the U phase voltage isapproximately equal to the COM voltage as the U phase voltage is falling(step 226). Once this occurs, the method 208 comprises determining the Vand W phase voltage states, as described above with respect to step 228(step 230). If the V and W phase voltage states are the same (i.e., bothHIGH or both LOW), the method 208 comprises concluding that the motor102 is in an abnormal state (step 232). If the V phase voltage state isHIGH and the W phase voltage state is LOW, the method 208 comprisesconcluding that the motor 102 is in positive rotation (step 234). If theV phase voltage state is LOW and the W phase voltage state is HIGH, themethod 208 comprises concluding that the motor 102 is in reverserotation (step 236).

FIG. 3 provides a table 300 that summarizes the possible outcomes ofsteps 224, 226, 228 and 230 in method 208 and lists the conclusionsabout motor state for each possible permutation of outcomes. The table300 includes column 302, which describes whether the U phase voltage isrising or falling; column 304, which describes the V phase voltagestate; column 306, which describes the W phase voltage state; and column308, which lists the corresponding conclusion about the state of themotor 102. Each of the rows 310, 312, 314, 316, 318, 320, 322, 324provides a different permutation of possible outcomes for steps 224,226, 228 and 230. Thus, for instance, referring to row 310, when the Uphase voltage is rising and the V and W phase voltage states aredetermined to both be LOW when the U phase voltage is equal or at leastapproximately equal to the COM voltage, the motor 102 is determined tobe in an abnormal state. Similarly, referring to row 320, when the Uphase voltage is falling and the V and W phase voltage states aredetermined to be LOW and HIGH, respectively, when the U phase voltage isequal or at least approximately equal to the COM voltage, the motor 102is determined to be in reverse rotation.

FIG. 4 shows two graphs 400, 450 that may be helpful in understandingthe method 208 of FIG. 2B. The graph 400 shows the COM voltage signal402 and the U, V, and W phase voltage signals when the motor 102 is inpositive rotation. Similarly, the graph 450 shows the COM voltage signal452 and the U, V and W phase voltage signals when the motor 102 is inreverse rotation. As can be seen in graph 400, as the U phase voltagesignal rises and falls, it regularly coincides with the COM signal 402.These points in time when the U phase voltage signal is equal or atleast approximately equal to the COM voltage signal are the times whenthe V and W phase voltage states are determined in method 208, asexplained above. Comparing graph 400 with table 300, for instance, thepoint in time marked by numeral 404 corresponds to row 322; numeral 406corresponds to row 312; numeral 408 corresponds to row 322, and numeral410 corresponds to row 312. Both row 312 and row 322 indicate positiverotation, as column 308 of table 300 indicates. Thus, the waveforms ingraph 400 indicate positive rotation of the motor 102. Referring tograph 450, at the point in time noted by numeral 454, the U phasevoltage signal is falling, while the V and W phase voltage states areLOW and HIGH, respectively. Thus, numeral 454 corresponds to row 320 intable 300, as does numeral 458. Likewise, numerals 456 and 460correspond to row 314. Both rows 314 and 320 indicate reverse rotation;thus, the graph 450 includes waveforms indicating reverse rotation ofthe motor 102.

The foregoing discussion of FIGS. 2B, 3 and 4 all pertain to theperformance of step 208 in the method 200 of FIG. 2A. The remainder ofthe method 200 is now described. If, at step 208, it is determined thatthe motor 102 is in positive rotation, the method 200 comprisesdetermining whether there is zero cross continuous detection (step 210).This means that the system determines the times at which the U, V and Wphase voltages intersect with the COM voltage and further determine notonly the time intervals between these intersections but also whethereach of the phase voltages is rising or falling at the intersections.Thus, for example, referring briefly to graph 400 in FIG. 4, if thesystem detects that the intersections with the COM voltage follow thepattern of U rising, W falling, V rising, U falling, W rising, Vfalling, U rising, W falling, etc. as shown, and further if the systemdetects that these intersections of the phase voltages with the COMvoltage follow predetermined timing intervals, then the systemdetermines with greater certainty that the motor is in positiverotation. Similarly, as mentioned with respect to step 212 below, if thesystem detects intersection behavior (e.g., order and timing) betweenthe phase voltages and the COM voltage as depicted in graph 450, thesystem determines with greater certainty that the motor is in reverserotation. If the intersection behavior is unrecognized or at least failsto fall into the patterns depicted in graphs 400 and 450, the motor maynot be rotating at all. In some embodiments, each instance of the zerocross continuous detection step depicted in the figures is optional. Insome embodiments, the precise order and timing requirements for thesystem to conclude that the motor is in fact in positive rotation orreverse rotation may be programmably varied as appropriate and desired.

If there is no zero cross continuous detection, the method 200 comprisesultimately concluding that the motor 102 is non-rotating and that itshould be treated as such (step 214). Otherwise, the method 200comprises ultimately concluding that the motor 102 is in positiverotation and that the electrical control start appropriate for positiverotation should be applied to the motor 102 (step 216). If, at step 208,it is determined that the motor 102 is in reverse rotation, the method200 comprises determining whether there is zero cross continuousdetection (step 212), as described above with respect to step 210. Ifnot, the method 200 comprises ultimately concluding that the motor 102is non-rotating and that it should be treated as such (step 214).Otherwise, the method 200 comprises ultimately concluding that the motor102 is in reverse rotation and that the electrical control startappropriate for reverse rotation should be applied to the motor 102(step 218). If at step 208 it is determined that the motor isnon-rotating (step 238), it is ultimately concluded that the motor 102is indeed non-rotating and it is treated as such (step 214). If, at step208 it is determined that the motor 102 is in an abnormal state (step232), the method 200 comprises return control of the method to step 206,as shown.

FIG. 5 is a flow diagram of another method 500 for identifying motorstate. The method 500 is virtually identical to the method 200 of FIG.2A. The difference is that in method 500, the number of times that themotor 102 is determined to be in an abnormal state at step 208 iscounted (step 502). If the count meets or exceeds a predeterminednumber, the method 500 comprises ultimately concluding that the motor102 is non-rotating (step 214). Otherwise, if the count does not meet orexceed the predetermined number, the method 500 comprises returning flowof the method 500 to step 206.

FIG. 6A is a flow diagram of yet another method 600 for identifyingmotor state. The method 600 is virtually identical to the method 200 ofFIG. 2A. The difference is that in method 600, a finding of abnormalstate at step 208 causes control of the method 600 to return to step 208instead of step 206.

FIG. 6B is a flow diagram of still another method 650 for identifyingmotor state. The method 650 is virtually identical to the method 500 ofFIG. 5. The difference is that in method 650, a determination that thenumber of times that the motor 102 has been found to be in an abnormalstate fails to meet or exceed a predetermined number (step 502) resultsin control of the method 650 returning to step 208 instead of to step206.

Numerous other variations and modifications will become apparent tothose skilled in the art once the above disclosure is fully appreciated.It is intended that the following claims be interpreted to embrace allsuch variations, modifications and equivalents. In addition, the term“or” should be interpreted in an inclusive sense.

What is claimed is:
 1. A method for determining the state of a brushlessmotor having first, second and third phases, comprising: decoupling saidmotor from a power source; determining whether said motor is rotating ornon-rotating; if the motor is rotating; determining a first phasevoltage state relative to a common voltage when a third phase voltage iswithin a predetermined range of said common voltage; and determining asecond phase voltage state relative to the common voltage when the thirdphase voltage is within the predetermined range of said common voltage;and if the first phase voltage state and the second phase voltage stateare the same, repeating said determination as to whether the motor isrotating or non-rotating.
 2. The method of claim 1, further comprising:if the first phase voltage state and the second phase voltage state arethe same, repeating said determination of the first phase and secondphase voltage states, wherein said determination of the first phase andsecond phase voltage states is repeated no more than a predeterminednumber of times.
 3. The method of claim 2, wherein, if saiddetermination of the first phase and second phase voltage states isrepeated said predetermined number of times, determining that the stateof said motor is non-rotating.
 4. The method of claim 1, furthercomprising: if the first phase voltage state is low and the second phasevoltage state is high, and if said first and second phase voltage statesare determined at a time when the third phase voltage is rising, thendetermining that the state of said motor is positive rotation.
 5. Themethod of claim 1, further comprising: if the first phase voltage stateis high and the second phase voltage state is low, and if said first andsecond phase voltage states are determined at a time when the thirdphase voltage is rising, then determining that the state of said motoris reverse rotation.
 6. The method of claim 1, further comprising: ifthe first phase voltage state is low and the second phase voltage stateis high, and if said first and second phase voltage states aredetermined at a time when the third phase voltage is falling, thendetermining that the state of said motor is reverse rotation.
 7. Themethod of claim 1, further comprising: if the first phase voltage stateis high and the second phase voltage state is low, and if said first andsecond phase voltage states are determined at a time when the thirdphase voltage is falling, then determining that the state of said motoris positive rotation.
 8. The method of claim 1, wherein determiningwhether the motor is rotating or non-rotating comprises: providing twocomparators, each of said comparators receiving the same phase voltageand each of said comparators receiving a differently biased version ofsaid common voltage, and if both outputs of said two comparators are thesame polarity, determining that the state of said motor is rotating. 9.A method for determining the state of a brushless motor having first,second and third phases, comprising: determining whether said motor isrotating or non-rotating; if the motor is rotating; determining a firstphase voltage state relative to a common voltage when a third phasevoltage is approximately equal to said common voltage; and determining asecond phase voltage state relative to the common voltage when the thirdphase voltage is approximately equal to said common voltage; and if thefirst phase voltage state and the second phase voltage state are thesame, repeating said determination of the first phase voltage state andthe second phase voltage state.
 10. The method of claim 9, furthercomprising disconnecting the motor from a power source prior toperforming said determination steps.
 11. The method of claim 9, wherein,if said determination of the first phase and second phase voltage stateshas been repeated a predetermined number of times, determining that saidstate of the motor is non-rotating.
 12. The method of claim 9, wherein,if the first phase voltage state and the second phase voltage state arethe same, omitting repetition of said determination as to whether themotor is rotating or non-rotating.
 13. The method of claim 9, furthercomprising: if the first phase voltage state is low and the second phasevoltage state is high, and if said first and second phase voltage statesare determined when the third phase voltage is rising, then determiningthat the state of said motor is positive rotation.
 14. The method ofclaim 9, further comprising: if the first phase voltage state is highand the second phase voltage state is low, and if said first and secondphase voltage states are determined when the third phase voltage isrising, then determining that the state of said motor is reverserotation.
 15. The method of claim 9, further comprising: if the firstphase voltage state is low and the second phase voltage state is high,and if said first and second phase voltage states are determined whenthe third phase voltage is falling, then determining that the state ofsaid motor is reverse rotation.
 16. The method of claim 9, furthercomprising: if the first phase voltage state is high and the secondphase voltage state is low, and if said first and second phase voltagestates are determined when the third phase voltage is falling, thendetermining that the state of said motor is positive rotation.
 17. Themethod of claim 9, wherein determining whether the motor is rotating ornon-rotating comprises: providing two comparators, each of saidcomparators receiving the same phase voltage and each of saidcomparators receiving a differently biased version of said commonvoltage; if both outputs of said two comparators are the same polarity,determining that the state of said motor is rotating.
 18. A system fordetermining the state of a motor, comprising: a motor that is associatedwith first, second and third phases and that is driven by a motordriving circuit; a control circuit that couples to and controls themotor driving circuit; a first comparator that receives a first biasedversion of a common voltage at one input and a first phase voltage, asecond phase voltage, or a third phase voltage at another input, anoutput of said first comparator provided to the control circuit; and asecond comparator that receives a second biased version of a commonvoltage at one input and the first phase voltage, the second phasevoltage, or the third phase voltage at another input, an output of saidsecond comparator provided to the control circuit; wherein, if theoutputs of the first and second comparators are both positive or areboth negative, the control circuit determines that the motor isrotating, wherein, if the motor is determined to be rotating, thecontrol circuit: determines a first phase voltage state relative to thecommon voltage when a third phase voltage is approximately equal to saidcommon voltage; and determines a second phase voltage state relative tothe common voltage when the third phase voltage is approximately equalto said common voltage, and wherein, if the first phase voltage stateand the second phase voltage state are the same, the control circuitrepeats said determination of the first phase voltage state and thesecond phase voltage state.
 19. The system of claim 18, wherein, if thefirst phase and second phase voltage states are the same, the controlcircuit repeats said determination of whether said motor is rotating.20. The system of claim 18, wherein first and second bias voltagesassociated with said first and second biased versions of the commonvoltage have opposing polarities and have absolute values that are lessthan: an absolute value of a maximum first phase voltage; an absolutevalue of a maximum second phase voltage; and an absolute value of amaximum third phase voltage.